One of the main functions of Streptococcus thermophilus strains used in the dairy industry is the production of lactic acid. In cheese and fermented milk manufacturing processes, the pH evolution kinetics must be reproducible in order to ensure the good quality of the final products. The objective of the present study was to investigate the effect of the metabolism of urea on the acidifying activity of fast- and slow-acidifying strains of S. thermophilus. Milk treatment with a purified urease and utilization of the urease inhibitor flurofamide revealed that urea metabolism by S. thermophilus influences the pH evolution kinetics through 2 distinct means. First, ammonia production from urea tends to increase the pH. This effect is greater when lactic acid concentration is low due to a lower buffering capacity of milk. Second, urea metabolism also modifies growth and lactic acid production by S. thermophilus. Depending on the strains and the growth stage of the cultures, consumption of urea induces either a faster or a slower pH decrease. For the slow-acidifying strain RD678, suppression of urea metabolism by adding flurofamide decreased the time necessary to reach pH 6 by 195 min. This effect was less pronounced for the 2 fast-acidifying strains RD674 and RD677. These results show that urea metabolism may have a considerable influence on the acidifying properties of S. thermophilus strains.

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Natural variations of the urea content of milk have a detrimental effect on the regularity of acidification by Streptococcus thermophilus strains used in dairy processes. The aim of the present study was to select urease-deficient mutants of S. thermophilus and to investigate their properties. Using an improved screening medium on agar plates, mutants were selected from 4 different parent strains after mutagen treatment and by spontaneous mutation. Most mutants were stable and had a phage sensitivity profile similar to that of their parent strain. Some of them contained detrimental secondary mutations, as their acidifying activity was lower than that of the parent strain cultivated in the presence of the urease inhibitor flurofamide. The proportion of this type of mutant was much lower among spontaneous mutants than among mutants selected after mutagen treatment. Utilization of urease-deficient mutants in dairy processes may have several advantages, such as an increase in acidification, an improved regularity of acidification, and a lower production of ammonia in whey.